Aluminum alloy sheets are light in weight as compared to a steel sheet and have good formability, and therefore, they have today taken the place of the steel sheet in sectors of body sheets for automotive vehicles, skeletal structures, ship components and the like. For its great strength and excellent formability, an alloy of an Al--Mg type (JIS Type 5000 series) has been proposed as typically applicable to the aluminum alloy sheets noted above.
The Al--Mg alloy, however, has the problem that upon lapse of a prolonged period of time after deforming, it tends to cause .beta. phase (Al.sub.2 Mg.sub.3) to preferentially precipitate as a form of film in its grain boundary, thus bringing about stress corrosion cracking. Various techniques have been found in solving this problem. For instance, Japanese Unexamined Patent Publication No. 4-187748 discloses a method of the production of an aluminum alloy sheet for automotive use having high resistance to stress corrosion cracking. The method comprises homogenizing an aluminum alloy ingot having Mg in a content of 3.5 to 5.5% by weight, hot-rolling and then cold-rolling the ingot, annealing the resultant sheet, without further cold rolling, and subjecting the annealed sheet to hold for 0.5 to 24 hours at a temperature of 150 to 230.degree. C. As a like instance, JP5-179413A or JP63-255346A discloses a method in which process comprises homogenizing aluminum alloy ingot after casting, hot-rolling and then cold-rolling the ingot, and annealing and slowly cooling the resultant sheet.
In order to improve the shape retention after deforming of an Al--Mg type alloy sheet, namely the shape fixability thereof, it is desired that the proof stress (or 0.2% yield strength) of such sheet be rendered to be as low as possible. To this end, a certain method is known as taught in Japanese Examined Patent Publication No. 6-68146. This prior art method contemplates cold-rolling a hot-rolled sheet or a continuously cast slab of an Al--Mg type alloy containing Mg in an amount of 2 to 6% by weight, and recrystallizing, quenching and solution heat treatment the cold-rolled sheet by means of quick heating and quick cooling, followed by annealing and correction treatment of the resultant sheet. In such method, when the heating temperature after correction is preset to range from 60 to 200.degree. C., heating and cooling is carried out at a rate of 4.times.10.sup.-3.degree. C./sec or above. In the case of the heating temperature at from 200 to 360.degree. C., heating and cooling are effected at a rate of 1.225.times.10.sup.-3 T-0.241.degree. C./sec or more where T denotes the heating temperature, this definition applying as such to the following instances. Alternatively, heat treatment is conducted for 10.sup.5 seconds or less in the case of the heating temperature at from 60 to 160.degree. C., for -5.33.times.10.sup.5 T+9.5.times.10.sup.5 seconds or less in the case of the heating temperature at from 160 to 175.degree. C., for -1.65.times.10 T+4.89.times.10.sup.4 seconds or less in the of the heating temperature at from 175 to 290.degree. C., and for -7.14 T+3.07.times.10.sup.3 seconds or less in the case of the heating temperature at from 290 to 360.degree. C. In that way, an aluminum alloy sheet is producible which is suitable for automotive use and has high strength and good formability.
However, the Al--Mg type alloy sheet obtained from continuous casting and rolling with use of the above cited method has the drawback that when heat-treated, it fails to attain sufficient resistance to stress corrosion cracking and adequate reduction in proof stress.